Elevator cars and other vehicles and devices, such as hooks, buckets and material harnesses on cranes or launching apparatus, are movable in two opposite directions, frequently by means of a cable or wire rope.
Generally speaking, elevator cars movable by hoist ropes are suspended by wire ropes which go over a traction sheave and down to a counterweight. The counterweight serves to reduce the power required to move the elevator and also to create traction (prevent slippage) with respect to the traction sheave. The traction sheave is driven directly by a motor or indirectly by a motor through a geared machine. A normal brake is applied to the drive to stop or hold the elevator at a floor.
With elevator cars, specifically, the usual elevator codes require that an emergency brake be included, such brake arresting the descent of the elevator car when it is descending at a speed in excess of a predetermined speed. A known braking device for such purpose is the safety device which grips the car guide rails even in the event of breakage of the elevator hoisting rope.
With a high factor of safety for the wire ropes, one country has recognized that these ropes never break and is allowing other emergency brakes in lieu of the safety device which grips the guide rails. Also, since counterweights are generally heavier than the elevator, with a mechanical failure, such as that of the normal brake, there is danger of the elevator overspeeding in the ascending direction In addition, depending on the load in the elevator car and with a mechanical failure, the car could leave the floor in either direction with the doors open. One country already requires emergency devices to be activated in the event of the above, and several other countries are considering code changes to require ascending car overspeed protection, as well as protection against leaving the floor with the doors opened. Known braking devices include brakes applied to the hoisting drum (traction sheave), to the hoisting ropes, or to the car or counterweight guide rails.
It is considered to be important that the braking force be substantially constant even with wear of various elements of the braking system, such as wear of the brake shoe linings.
Braking apparatus which will stop an elevator when it overspeeds in either direction is known in the art and includes brake elements applied to the hoisting (suspension) ropes by air actuated means. While such apparatus will maintain the braking pressure constant with brake shoe lining wear, the apparatus includes several elements, such as hoses, tanks and an air cylinder or air compressor, which are subject to failure which can render the braking inoperative.